Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations flag destructiveHint=true and idempotentHint=true, so the safety profile is already structured; the description adds real behavioral context on top: partial update semantics, deep-merge behavior for renderParams, and the side effect that a scope change (url/selector/fullPage/size/device) discards the baseline and re-baselines on the next check. It stops short of stating whether re-baselining loses diff history or what errors occur on invalid merges.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

It is a single dense paragraph that front-loads the capability list before the semantics. Every clause carries information (partial update, deep merge, re-baseline, return type), though the opening sentence is packed enough that a bulleted split would scan faster.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For an 11-parameter tool with a nested renderParams object and no output schema, the description covers the mutation model, merge behavior, destructive side effects, and the return value ("Returns the updated watch as JSON"). Remaining gaps are minor: no mention of authorization/plan failures beyond what per-parameter schema descriptions say, and no error or conflict behavior.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so the baseline is 3, but the description adds meaning the schema does not state as such — that only supplied fields are mutated (partial update), that renderParams is deep-merged rather than replaced, and that nulling webhookUrl/notifyEmail disables the channel. Per-parameter constraints and plan gating remain in the schema, which is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description opens with a specific verb and resource ("Update a watch in place") and then enumerates the concrete mutation classes: pause/resume, re-point, schedule/diff/notify settings, and disabling delivery channels. An agent can distinguish this from watch_create, watch_delete, or watch_test without opening any schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

It establishes the key precondition for use — partial update semantics ("Only the fields you send change") — and explains the two non-obvious use cases (null to turn a channel off, scope change to re-baseline). It does not explicitly route away to siblings such as watch_test (to verify before committing) or watch_delete (to remove), so it stops short of full when-not guidance.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.